Investigating the corrosion behavior of biodegradable Mg–5Zn alloy coated with hydroxyapatite reinforced composite fabricated by friction stir process

Abstract

In this study, developing a composite coating on the surface of a biodegradable Mg–5Zn alloy using friction stir process (FSP) is the goal to control biodegradation of the alloy. The experimental procedure starts with the alloy casting, followed by homogenization treatment. Rotational speeds of 800, 1000, and 1250 rpm and 1, 2, and 3 passes using hydroxyapatite (HA) powder are utilized to develop Mg–5Zn/HA composite coating on the surface of the alloy. Microstructure of the as-cast Mg–5Zn alloy and Mg–5Zn/HA composite coating is characterized. Biodegradation behavior of the as-cast and coated alloy is also assessed in simulated body fluid using electrochemical potentiodynamic polarization and impedance spectroscopy tests. The results show that FSP can always modify the surface layer through the dynamic recrystallization process, leading to at least 35% decrease in corrosion rate of the alloy. The potentiodynamic polarization curves of the alloys disclose a break, indicating the impact of passivating behavior. The results also reveal that the rotation speed of 1000 rpm, with the increase in the number of passes, generates more refined microstructure on the surface of Mg–5Zn alloy, which improves its corrosion resistance with decreasing the corrosion rate of the alloy from 28.64 mpy to 0.27 mpy after FSP for three passes at 1000 rpm. In contrast, FSP with the rotation speed of 800 rpm and one pass, besides providing the refined microstructure, develops better distribution of hydroxyapatite particles on the surface, which results in the highest corrosion resistance among the FSPed Mg–5Zn/HA composite coated alloy.

Keywords

• Magnesium alloy
• Friction stirring process
• Composite coating
• Biodegradation
• Hydroxyapatite